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Old 23rd Oct 2019, 9:25 pm   #1
Karen O
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Default Computer retro homebrew: Orton ZX79

Here's something I've been working on for the last month. It's my version of the ZX80, but using only ten standard parts

I've got it to the stage of being a 'TV typewriter' i.e. the terminal BIOS is done. Now I have to shoehorn a BASIC into the remaining ROM. I'm not really into BASIC. I wonder if I'd do better to add machine code development tools instead...?

The circuit to the right (in the white open case) is a ROM emulator that I built. It really speeds up development (especially as I have no EPROM programmer yet, so it could be said to speed things up infinitely!)

When it's finished I might add a serial expansion board and get it to drive my little printer
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Old 24th Oct 2019, 2:21 am   #2
Argus25
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Default Re: Computer retro homebrew: Orton ZX79

Wow ! That is a very cool project. There have been some very code compact versions of BASIC in the past to run with limited memory in early S-100 & other computers, one was called "TINY BASIC".
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Old 24th Oct 2019, 4:45 am   #3
TonyDuell
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Default Re: Computer retro homebrew: Orton ZX79

Why have BASIC at all? I'd go for FIG-Forth.
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Old 24th Oct 2019, 11:24 am   #4
Karen O
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Default Re: Computer retro homebrew: Orton ZX79

I'd like to turn my hand to language development. I've written an expression parser before (real time computation in DSP application) so I'm sure I can do it.

There's no real need to develop on the hardware though. What I need is a Z80 simulator for DOS where I can intercept terminal input and output, and redirect this to a PC simulation of my computer's screen and keyboard running in a DOS window.

Porting an existing BASIC won't be helped by my computer's non-standard character set.
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Old 29th Oct 2019, 12:35 am   #5
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Default Re: Computer retro homebrew: Orton ZX79

Excellent. What chips are in there and how much ROM space is left for BASIC. I ported BBC BASIC to the 6809 (not 100% finished by useable). It's quite large but then it does contain a full 6809 assembler too!

They keyboard looks cool - how is it constructed?

The z80 port is open source and I'm sure could be cut down to be quite small if you sacrificed, say, the assembler, floating point and specific graphics instructions etc.

D
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Old 29th Oct 2019, 7:18 am   #6
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Default Re: Computer retro homebrew: Orton ZX79

Hi Dom,
The main chips are Z80A, 8k ROM (half used for font tables) and 2k RAM. The other 7 chips are LSI and MSI LS TTL. The keyboard is tact switches with a laminated sheet on top. It works okay and you get a definite click unlike membrane keyboards.

I've got a program editor working! I can't run anything yet though

In the process I've conquered a number of syntactic elements - parse number, keyword, print number, etc. The fun part (the actual language) can start now.

Progress has been quick despite lack of debug facilities.
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Old 30th Oct 2019, 11:33 pm   #7
Colin Ames
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Default Re: Computer retro homebrew: Orton ZX79

Nice work Karen.
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Old 31st Oct 2019, 8:52 am   #8
Karen O
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Default Re: Computer retro homebrew: Orton ZX79

Thank you, Colin.

I'm working on the cassette routines now.
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Old 11th Nov 2019, 4:56 am   #9
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Default Re: Computer retro homebrew: Orton ZX79

I've had my new computer running simple programs!

I've also got the cassette interface and the expression evaluator working.

Just got GOTO, GOSUB, RETURN, FOR, NEXT statements to do now.
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Old 11th Nov 2019, 12:10 pm   #10
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Default Re: Computer retro homebrew: Orton ZX79

Did you go for tokenized or plain text?
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Old 11th Nov 2019, 12:16 pm   #11
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Default Re: Computer retro homebrew: Orton ZX79

It's just plain text, Dom.

I've based it on NIBL BASIC which is as simple as it comes.
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Old 11th Nov 2019, 3:21 pm   #12
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Default Re: Computer retro homebrew: Orton ZX79

ZX79? Ah, the old one-step-ahead-of-IBM thing.

Having the name David, I'm just a little bit wary of that sort of computer. I'll open my own pod bay doors, thank you.

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Old 11th Nov 2019, 6:03 pm   #13
Karen O
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Default Re: Computer retro homebrew: Orton ZX79

Don't worry Dave, you can sit down calmly, take a stress pill and think things over

Yeah, the '79' thing indicates that it's even more regressive than the ZX80!
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Old Yesterday, 2:33 pm   #14
Karen O
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Default Re: Computer retro homebrew: Orton ZX79

Well, it's about finished. I've got about 300 bytes left so there's room for patches/extensions. I've even got (shortened) error messages e.g. 'SNTX ERROR IN 50'.

Next step is to put the BASIC on EPROM and try running it independently of the ROM emulator. There'll be a few bugs of course, but so far everything checks out
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